In the drying of sheet materials that have been coated with a layer of liquid coating composition, it is a common practice to utilize a drying apparatus in which a gaseous drying medium, usually air that has been heated to a suitable elevated temperature, is brought into direct contact with the coated layer in order to bring about evaporation of the liquid medium from the layer. In such driers, the gaseous drying medium is directed in a manner which distributes it uniformly over the surface of the coated layer under carefully controlled conditions that are designed to result in a minimum amount of disturbance of the layer. A common type of drier utilizes a plenum into which the gaseous drying medium is admitted and from which the gaseous drying medium is discharged through a multiplicity of holes, slots or nozzles onto the surface of the layer which is to be dried. In the operation of such driers, the sheet material, which is typically in the form of a web, is continuously conveyed through the drier along a predetermined path at a suitable rate commensurate with the drying load and the operating conditions utilized; while spent gaseous drying medium--that is, gaseous drying medium which has become laden with vapor evaporated from the layer of coating composition--is continuously discharged from the drier. As the web travels through the drier, the gaseous drying medium is directed from the plenum onto the coated surface and the spent medium flows away from the path of travel to be discharged.
A wide variety of different drier designs are known to the art. Thus, for example, the drier can be designed so that the flow of spent gaseous drying medium is essentially transverse to the path of travel of the web, i.e., the spent medium flows over the edges of the web so as to exit from the drier, or so that the flow of spent medium is essentially perpendicular to the path of travel of the web. Also, while it is usually most convenient for the sheet material to be in the form of a web, it can instead be in the form of a succession of discrete sheets conveyed through the drier by suitable means such as an endless belt.
The drying of sheet materials which have been coated with two or more superposed layers is carried out in the same manner as is described above in reference to a single layer coating. To facilitate description, reference is frequently made herein to the coating and drying of a "layer" of coating composition, but it is to be understood, unless the context otherwise requires, that the discussion applies also to the coating and drying of two or more superposed layers. Moreover, the method and apparatus of the present invention find utility not only in manufacturing operations involving wet-on-wet coating techniques, but also in manufacturing operations involving sequential coating and drying steps. As will be readily understood by those skilled in the coating art, wet-on-wet coating techniques include simultaneous multi-layer coating methods, in which two or morre distinct layers are applied to a web support at the same time and the resulting multi-layer composite is dried, and methods in which distinct layers are applied separately, but in close succession and the resulting multi-layer composite is dried, i.e., a first layer is coated on the web and then a second layer is coated over the first layer while it is still in a wet state, and so forth. In contrast, in operations involving sequential coating and drying steps, a first layer is coated and dried, a second layer is coated over the first layer and dried, and so forth.
One of the most common and difficult to avoid problems that is encountered in the drying of coating compositions is the formation of mottle. It is a problem that is encountered under a wide variety of circumstances. For example, mottle, or non-uniform density, is frequently encountered when compositions consisting of solutions of a polymeric resin in an organic solvent are coated in layer form onto sheet materials, such as webs of synthetic organic plastic material. Mottle is an especially severe problem when the coating solvent is a volatile organic solvent but can occur to a significant extent even with aqueous coating compositions or with coating compositions utilizing an organic solvent of low volatility. The mottle is an undesirable defect in some instances because it detracts from the appearance of the finished product and in some instances, such as in the photographic art, it is also undesirable because it adversely affects the functioning of the coated article. Various expedients have been employed heretofore in an effort to eliminate, or at least minimize, the formation of mottle in coated layers. For example, surfactants are often added to the coating compositions as described, for example, in U.S. Pat. No. 3,514,293. These are sometimes effective in reducing mottle, but in many cases the degree to which mottle forms is still excessive in spite of the inclusion of a surfactant in the coating composition. It is believed that there are a variety of factors which can contribute to the formation of mottle and the exact mechanism of its formation is not well understood. Regardless of the specific causes of mottle, its formation in coated layers, as well as the occurrence of other defects such as streaks and lines, is a long standing problem of serious concern in the manufacture of coated materials, and especially in the manufacture of photographic products.
Among the factors which contribute to mottle formation in the drying process are non-uniform drying conditions that commonly exist in driers of the type described hereinabove. Thus, for example, turbulent flow conditions within the gaseous drying medium can result in physical disturbance of the coated layer that manifests itself as mottle in the dried product. Also, non-uniformities with respect to temperature, with respect to heat transfer rates, and with respect to the concentration of vapor in the gaseous drying medium, lead to non-uniform rates of evaporation at different points within the coated layer. The cooling which results from evaporation causes the temperature at the surface of the coated layer to decrease, so that variation in the rate of evaporation leads to the establishment of temperature differences within the layer. Such temperature differences are believed to function to induce convective flow in the layer which is a significant factor in contributing to the formation of mottle. Particular difficulty in this regard is caused by the flow of the spent gaseous drying medium in direct contact with the surface of the coated layer.
The present invention is directed toward the objective of providing an improved method and apparatus for drying coated sheet materials which reduces or eliminates many of the deficiencies in known drying methods and apparatus that contribute to the formation of mottle.